This invention relates to a process for producing a starch composition, to a paper furnish comprising the starch composition, to a process for using the starch composition as retention aid in papermaking and to a process for using the starch composition as clarifying aid for removing solids from an aqueous dispersion.
Paper production involves the formation and dewatering of a web primarily composed of cellulose fibers and inorganic filler. The web is formed by spreading an aqueous suspension containing the cellulose fibers and inorganic filler over a wire or net, and then removing water to form a fiber web or sheet. The aqueous suspension is referred to as xe2x80x9cpaper furnishxe2x80x9d in the trade, and the removed water is referred to as xe2x80x9cwhite water.xe2x80x9d
The industry long has sought ways to reduce the percentage of small cellulose fibers and filler particles that are removed with the white water as the paper web is formed. Not only does this represent a loss of material, but it also contributes to a build-up of material in the white water known as xe2x80x9canionic trashxe2x80x9d that impairs efficient operation of the equipment. Thus, improved retention of the smaller particles not only makes water removal easier, but also improves yield and productivity of the papermaking process.
Many additives have been suggested in the prior art for improving fines retention and wet-end drainage. Cationic starches frequently are used for this purpose, particularly the relatively expensive cationic potato and waxy maize starches. Less expensive cationic corn starch has been used, but it generally does not provide adequate fines retention and wet-end drainage.
Many suggestions are made in the prior art concerning improvements to the efficiency of starches for this purpose. U.S. Pat. Nos. 5,859,128 and 6,048,929 disclose a modified starch wherein the starch is prepared by cooking the starch in the presence of a nonionic, amphoteric, or cationic polyacrylamide. The modified starch exhibits improved performance as a retention additive in the manufacture of paper. U.S. Pat. No. 5,928,474 discloses a paper furnish containing a modified starch such as prepared in the above two patents. U.S. Pat. No. 6,033,525 discloses a paper furnish containing a modified starch such as prepared above and a soluble aluminum compound wherein the performance of the modified starch is enhanced by the presence of the aluminum compound in the furnish.
The process disclosed above require adding an alkaline aluminum compound such as, for example, sodium aluminate and an anionic aluminum compound to polyacrylamide in the cooking step to improve retention. However, the cost of sodium aluminate is relatively high compared to other basifying agents. There is a need for improving fines retention and wet-end drainage in papermaking without the use of sodium aluminate to produce a modified starch.
A process that can be used for producing a starch product is provided.
The process comprises heating a composition, which comprises or consists essentially of a starch, a polyacrylamide, and a multivalent cation in which the weight ratio of starch to polyacrylamide is greater than about 2 to 1. The process can also comprise heating a composition, which comprises or consists essentially of a starch and a polyacrylamide to produce a heated composition followed by contacting the heated composition with a multivalent cation. The heating can be carried out at a pH above 7.0 if the polyacrylamide is a cationic polyacrylamide or nonionic polyacrylamide and, if the starch is a cationic starch and the polyacrylamide is an amphoteric polyacrylamide or anionic polyacrylamide, the heating neutralizes less than 75% of the cationic starch.
The term xe2x80x9cheatingxe2x80x9d used herein is exchangeable with xe2x80x9ccookingxe2x80x9d.
The starch can be any of those known to one skilled in the art for use in papermaking. It is preferably an amphoteric starch or cationic starch. A cationic starch can be derived from any common starch producing materials such as corn starch, potato starch, waxy maize starch, wheat starch, and combinations of two or more thereof. It can be obtained by any means known to one skilled in the art such as, for example, cationization by addition of 3-chloro-2-hydroxypropyltrimethylammonium chloride to obtain cationic starches with various degrees of nitrogen substitution. The degree of cationic substitution on the starches (weight % nitrogen/starch) can range from about 0.01 to about 0.2, preferably 0.02 to 0.15. Examples of naturally occurring amphoteric starches include potato starch. Synthetic amphoteric starches can also be used.
Any polyacrylamide known for use in papermaking can be used. It includes cationic polyacrylamide, anionic polyacrylamide, amphoteric polyacrylamide, and nonionic polyacrylamide. The preferred polyacrylamide is a cationic polyacrylamide having a molecular weight of at least 500,000, preferably at least 1,000,000. A suitable cationic, anionic or amphoteric polyacrylamide can have a degree of substitution by weight from 1% to about 80%, preferably from 10% to about 40%. Degree of substitution means that the polymers contain a certain degree or percent of randomly repeating monomer units containing chemical functionality which become charged when the polymer is dissolved in water. For example, the monomer units in cationic polyacrylamides or amphoteric polyacrylamides include, but are not limited to, groups such as amine groups. The polyacrylamide can be a solid, powder form, micro-bead form, a water-in-oil emulsion or any other commercially known form. Suitable polyacrylamides can be obtained from Allied Colloids, Suffolk, Va., and from Nalco, Naperville, Ill. as well as other sources.
Though a variety of multivalent cations can be used, it is preferred to use a multivalent cation that does not interfere with a papermaking process or an end use of the paper product. These cations include, but are not limited to, calcium cation, magnesium cation, iron cations, aluminum cations, zinc cation, titanium cations, cobalt cations, nickel cation, copper cation, barium cation, zirconium cation, manganese cation, and combinations of two or more thereof. The preferred cations are calcium cation, magnesium cation, and combinations thereof. The cations can also be combined with any suitable anion such as, for example, sulfate, nitrate, chloride, carbonate, phosphate, borate, hydroxide, oxide, and combinations of two or more thereof, provided that the resulting combination does not interfere with a papermaking process or an end use of a paper product.
A starch, a polyacrylamide, and a multivalent cation can be combined by any means known to one skilled in the art such as, for example, dry blended together or blended as a slurry or slurries in water. They can be combined before being heated. Polyacrylamide can be blended with a starch when the starch is being heated. A multivalent cation can also be introduced any time to produce the composition. A polyacrylamide, before being blended and/or heated with a starch can also be prehydrated, i.e., mixed with water prior to its combination with the starch. Wishing not to be bound by theory, it is believed that prehydration causes a polyacrylamide molecule, initially in the form of a coil, to unwind and become more soluble and/or reactive. Rather than blending dry or slurried starch, the starch can instead be heated or cooked, blended with the polyacrylamide, and then re-heated or re-cooked. Heating can be carried out by any means known to one skilled in the art such as using a starch cooker at a paper mill. A batch cooker or continuous cooker, such as a jet cooker, can be used. Continuous jet cooking can be conducted at temperatures from about 80 to 130xc2x0 C. at 1 atmosphere or higher pressure or at any temperature that can accommodate pressure. The solids content during cooking generally is less than 15%, but higher solids concentrations can be used.
Heating is generally carried out under a sufficient condition to gelatinize the starch and to at least partially hydrate a polyacrylamide and the hydrated react with the starch. Elevated temperatures above 60xc2x0 C., preferably above 65xc2x0 C., and more preferably 80xc2x0 C. to 100xc2x0 C. can be used. Temperatures below 60xc2x0 C. can also be used for some compositions and cooking equipment. Temperatures above 100xc2x0 C., for example, as high as 130xc2x0 C., or higher can be used if decomposition of the starch and polyacrylamide is prevented under a suitable pressure. Generally it can be as short as a few minutes to about 2 hours. Longer times generally are required for lower temperatures.
When using cationic or nonionic polyacrylamide, best results are obtained if the starch/polyacrylamide mixture is cooked at a pH above 7, although some improvement in ash retention is also found at a pH below 7.
The preferred cooking pH for either cationic or nonionic polyacrylamide and starch is from about 8 to about 10.5. Cooking pH is not critical when an anionic or amphoteric polyacrylamide is used, but typically can be in the range of 3 to 11. Cooking pH can be adjusted with conventional acids, bases, or salts.
The weight ratio of starch to PAM can vary with selected starch and polyacrylamide, and the extent to which improvements are desired in fines retention and wet end drainage, but is preferably above 2 to 1, preferably greater than 5 to 1, and more preferably greater than 10 to 1. For example, it has been found that poor-performing corn starch may be improved to the extent that it is equal or superior to the more expensive potato starch by cooking the corn starch with as little as 1% polyacrylamide, by weight. The amount of polyacrylamide added to the starch is preferably limited below that which can cause the starch to precipitate, which varies with the starch, polyacrylamide, and/or multivalent cation used.
The multivalent cation can be combined with a starch, a polyacrylamide, or a combined starch/polyacrylamide mixture at any time before, during, or after the heating step. It is preferably added before or during cooking. The amount of cation can be in the range of about 0.001 to about 20, preferably about 0.01 to about 15, and most preferably about 0.1 and about 10 parts by weight of cation per part of polyacrylamide.
The process is preferably carried out under moderate to low shear conditions. The composition produced by the process is also preferably stored under such conditions. That is, the agitation used is preferably adequate to mix the ingredients together and to aid in heat transfer requirements, but not produce an unnecessarily high shear which can degrade the effectiveness of a starch/polyacrylamide composition. The conditions can include lower than about 500 rpm (revolution per minute), preferably lower than about 450 rpm, more preferably lower than 350 rpm, and most preferably lower than 250 rpm. Also preferred is no agitation at all during cooling and/or storage.
The cooked starch/polyacrylamide composition, referred to herein as modified starch, can be added to any suitable paper furnish as a retention aid to improve fines retention and wet end drainage. The paper furnish can contain a variety of wood pulp and inorganic fillers, and typically has a pH of about 4 to 10. For example, bleached kraft pulp, thermomechanical, chemical-thermomechanical or groundwood pulps can be used together with clays, precipitated or ground calcium carbonate, titanium dioxide, and other inorganic fillers if desired. Such fillers typically are used at the 15% to 20% loading level, as a weight percent of the total paper weight, but can reach higher levels such as 30% or higher, for some specialty applications.
Particularly advantageous results can be obtained when the paper furnish also contains an anionic inorganic colloid, as is conventional in the papermaking industry. Thus the furnish can contain, for example, montmorillonite, bentonite, silica sols, aluminum modified silica sols, aluminum silicate sols, polysilicic acid, polysilicate microgels and polyaluminosilicate microgels, separately or in combination.
The paper furnish also can contain other typical additives, such as size, aluminum compounds (alum, aluminates, polyaluminum chlorides, etc.), cationic polymers (retention aids and flocculants), anionic polymers, and/or separate additions of starch. Aluminum compounds in particular have been found to boost retention performance of the cooked starch/polyacrylamide compositions. While the above additives can be added in any, the preferred order is to add the aluminum compound first, the cooked starch/polyacrylamide of this invention, and then an inorganic anionic colloid.
Generally, a paper furnish can contain, based on total dry weight of the paper furnish, at least about 0.01 and preferably between 0.1-50 kg/tonne of the starch/acrylamide (modified starch) of the invention; at least about 0.01 and preferably between 0.1-40 kg/tonne of an anionic inorganic colloid; and optional aluminum compounds of about 0.01 to 20 kg/tonne on an Al2O3 basis.
While the invention has been described in detail as applied to papermaking, the compositions produced by the invention process also have utility as clarifying aids to remove solids from aqueous suspensions.
The invention now will be exemplified, but not limited, by the following Examples.